This invention relates to systems for collecting and condensing electromagnetic radiation, particularly a system incorporating substantially ellipsoidal reflectors for collecting radiation emitted from a radiation source and focusing the collected radiation onto a target.
An optical collecting and condensing system comprises various optical elements, such as reflectors and lenses, that receives light energy from a light source, such as a light bulb, and directs the light energy toward a target. In particular, the optical system collects and condenses electromagnetic radiation to couple the light energy to a standard waveguide, such as a single fiber or fiber bundle or to output the light energy to a homogenizer of a projector. The functional objective for the optical system is to maximize the brightness (i.e., the flux intensity) of the electromagnetic radiation at the target.
Optical systems for collecting and condensing light from a light source are generally classified as either xe2x80x9con-axisxe2x80x9d or xe2x80x9coff-axis.xe2x80x9d In the on-axis systems, the reflectors are positioned on the optical axis between light source, and the target. FIG. 1 illustrates a known on-axis optical system that uses a paraboloid reflector with an imaging lens. The paraboloid reflector has the feature that light energy emanating from a focus is substantially collimated to travel parallel to the optical axis. The optical system of FIG. 1 uses this feature of the paraboloid reflector by positioning the light source at the focus in order to collimate the light from the light source. A condensing lens positioned in the optical stream receives the substantially collimated light energy and redirects the light energy toward the target. In this way, the light energy is collected and condensed at the target. The use of the paraboloid reflector further allows the use of various types of optical filters to improve the performance and durability of the optical system. However, the divergence of the light varies continuously along the reflector, with rays traveling near the optical axis having the greatest divergence. As a result, the magnification of the system varies along the different paths taken by the light emitted from the light source, causing degradation of the brightness of the system. Moreover, the focusing lens produces a distorted image even under perfect conditions and under actual operation typically produces badly aberrated images which effectively increase the image size and reduce flux intensity at the target.
FIG. 2 illustrates another known on-axis optical system. This system uses an ellipsoidal reflector, which has the feature that all light emanating from one focal point is directed to a second focal point. The optical system of FIG. 2 uses an ellipsoidal reflector with a light source placed at the first focus and a target placed at the second focus. As in the previous system, the on-axis ellipsoidal system suffers from brightness degradation caused because the divergence of the light varies continuously along the reflector, with rays traveling near the optical axis having the greatest divergence.
Overall, on-axis systems generally suffer from the basic limitations of losing brightness in the coupling, thus degrading the overall efficiency of the optical illumination and projection system. In particular, the divergence of the reflected beam in known on-axis systems is undesirably dependent on the angle of emission from the radiation source. Additionally, the outputs of the on-axis system are substantially circular and symmetric and, therefore, may not be suitable for non-circular targets, such as a rectangular homogenizer for use in projection.
In off-axis optical collecting systems, the reflectors are positioned off the optical axis between the light source and the target. For example, FIG. 3 illustrates an optical system in which the light source is positioned at a focal point of a retro-reflector and the target is positioned on a focal point of a primary reflector, but the reflectors are positioned off the optical axis between the light source and the target. In the illustrated optical system, light energy from the light source reflects from the retro-reflector and travels to the primary reflector. The optical energy then reflects from the primary reflector and converges at the target.
With the off axis system of FIG. 3, the magnification is very close to 1-to-1 for all angles of light when the numerical aperture of the system is small. When the system uses mirrors having higher numerical apertures (e.g., attempts to collect more light energy from the same light source) the larger angle light rays are reflected with high divergence angles, causing the magnification to deviate from 1-to-1. Again, the magnification reduces the brightness at the target and overall decreases the performance of the optical system. The amount of deviation in the magnification depends on the size of the mirror, the radius of curvatures, and the separation of the arc lamp and the target. Accordingly, the off-axis configuration of FIG. 3 is more suitable for applications that use smaller numerical apertures.
Different off-axis optical systems are also known. For example, U.S. Pat. No. 4,757,431 (xe2x80x9cthe ""431 patentxe2x80x9d) provides a condensing and collecting system employing an off-axis spherical concave reflector which enhances the maximum flux intensity illuminating a small target and the amount of collectable flux density by the small target. Enhancements to the optical system of the ""431 patent are provided by U.S. Pat. No. 5,414,600 (xe2x80x9cthe ""600 patentxe2x80x9d), in which the off-axis concave reflector is an ellipsoid, and by U.S. Pat. No. 5,430,634 (xe2x80x9cthe ""634 patentxe2x80x9d), in which the off-axis concave reflector is a toroid. Although the toroidal system described in the ""634 patent corrects for astigmatism, and the ellipsoidal system of the ""600 patent provides a more exact coupling than the spherical reflector of the ""431 patent, each of these systems requires the application of an optical coating onto a highly curved reflective surface, which is relatively expensive and difficult to apply in a uniform thickness.
Overall, the known off-axis optical systems provide a generally near 1-to-1 (i.e., magnification free) image of the light source at the target and conserve brightness. However, in the known off-axis systems, the magnification deviates from 1-to-1 as the amount of light collected is increased by increasing the collection angle of the reflector. Thus, as a greater portion of light energy from a light source is collected to increase optical intensity, the overall performance of the optical system degrades.
To address problems in the known optical collection and condensing systems, U.S. patent application Ser. No. 09/604,921 provides an on-axis, dual-paraboloid reflector system that is advantageous in many respects to other known systems, including the achievement of near 1-to-1 magnification for small-sized light sources. This optical collection and condensing system, as illustrated in FIG. 4, uses two generally symmetric paraboloid reflectors that are positioned so that light reflected from the first reflector is received in a corresponding section of the second reflector. In particular, light emitted from the light source is collected by the first parabolic reflector and collimated along the optical axis toward the second reflector. The second receives the collimated beam of light and focuses this light at the target positioned at the focal point.
To facilitate the description of this optical system, FIG. 4 includes the light paths for three different rays (a, b, and c) emitted from the light source. Ray a travels a relatively small distance before intersecting the first parabolic reflector, but the divergence of ray a at the first parabolic reflectors is relatively large. In contrast, ray c travels further between the light source and the first parabolic reflector but has a smaller relative divergence at the first reflector. Ray b, positioned between rays a and c, travels an intermediate distance before intersecting the first parabolic reflector and has an intermediate divergence. In this optical system, due to the symmetry of the two parabolic reflectors, the rays a, b, and c are reflected at corresponding positions in the second parabolic reflector such that the distance for each ray between the second parabolic reflector and the target is the same as the distance between the light source and the first parabolic reflector. In this way, the second reflector compensates for the divergence. Consequently, the optical system collects and condenses light energy from the light source with a near 1-to-1 magnification and preserves the brightness of the light source.
The optical system of FIG. 4 may further employ a retro-reflector in conjunction with the first paraboloid reflector to capture radiation emitted by the source in a direction away from the first paraboloid reflector and reflect the captured radiation back through the source. In particular, the retro-reflector has a generally spherical shape with a focus located substantially near the light source (i.e., at the focal point of the first paraboloid reflector) toward the first paraboloid reflector to thereby increase the intensity of the collimated rays reflected therefrom.
One shortcoming in the above described on-axis, dual-paraboloid optical system arises because the light source is very close to the apex side of the reflector. As a result, the system produces a large angle of divergence near the light source (i.e., along the paths similar to ray a). In particular, a large angle of divergence causes light energy traveling along a path similar to ray a to compass a relatively larger area on the second parabolic reflector, thus producing unwanted aberrations and a loss of brightness.
In view of these shortcomings in known optical collection and condensing systems, there remains a present need for an improved method and system of coupling light from a small source to illumination and projection systems.
In response to the above-identified needs, the present invention provides an improved optical collection and condensing system. The improved system for collecting and condensing electromagnetic radiation employs opposing reflectors to achieve unit magnification, or near unit magnification, between a source image and a focused image at a target, thereby producing maximum focused intensity at the target. In particular, the present invention is directed to an optical device for collecting electromagnetic radiation from a source of electromagnetic radiation and focusing the collected radiation onto a target to be illuminated with at least a portion of the electromagnetic radiation emitted by the source. The device includes first and second reflectors, where each reflector generally comprises at least a portion of an ellipsoid of revolution and has an optical axis and two focal points on the optical axis. A light source located proximate the one of focal points of the first reflector produces rays of radiation that converge at the second focal point of the first reflector. The second reflector is positioned and oriented with respect to the first reflector so that the rays of radiation reflected from the first reflector converge at a one focal point of the second reflector. The rays of radiation then continue until reflected by the second reflector and focused toward a target located proximate a second focal point of the second reflector. The first reflector and the second reflector have substantially the same size and shape and may be oriented optically about symmetrically with respect to each other so that each ray of radiation reflected by a surface portion of the first reflector is reflected by a corresponding surface portion of the second reflector toward the target to achieve a unit magnification.
A retro-reflector may be used in conjunction with the first reflector to capture radiation emitted by the source in a direction away from the first reflector and to reflect the captured radiation back through the source (i.e., through the first focal point of the first reflector) toward the first reflector to thereby increase the intensity of the rays reflected therefrom.
The shape of the first and second reflectors may deviate from an ellipsoid as needed by the system. Similarly, the first and second reflectors may have a toroidal or spherical shape that approximate an ellipsoid.